SOD is an enzyme which catalyzes the dismutation of free superoxide (O.sub.2.sup.-) to give molecular oxygen and hydrogen peroxide as follows: EQU 2 O.sub.2.sup.- +2H.sup.+ .fwdarw.O.sub.2 +H.sub.2 O.sub.2.
The resulting peroxide is then converted by catalases or peroxidases into H.sub.2 O. Superoxide (O.sub.2.sup.-) is a toxic by-product of aerobic respiration and reacts with peroxide (O.sub.2.sup.2-) to cause damage to proteins, lipids and nucleic acids. The enzyme superoxide dismutase (SOD) counteracts the toxic effects of the radicals by conversion of superoxide to molecular oxygen and peroxide.
The term "polymerase chain reaction" or "PCR" refers to a process of amplifying one or more specific nucleic acid sequences, wherein (1) oligonucleotide primers which determine the ends of the sequences to be amplified are annealed to single-stranded nucleic acids in a test sample, (2) a nucleic acid polymerase extends the 3'-ends of the annealed primers to create a nucleic acid strand complementary in sequence to the nucleic acid to which the primers were annealed, (3) the resulting double-stranded nucleic acid is denatured to yield two single-stranded nucleic acids, and (4) the processes of primer annealing, primer extension and product denaturation are repeated enough times to generate easily identified and measured amounts of the sequences defined by the primers. Practical control of the sequential annealing, extension and denaturation steps is exerted by varying the temperature of the reaction container, normally in a repeating cyclical manner. Annealing and extension occur optimally in the 37.degree. C.-80.degree. C. temperature range (exact value depending on primer concentrations and sequences), whereas denaturation requires temperatures in the 80.degree.-100.degree. C. range (exact value depending on target sequence and concentration).
The term "oligonucleotide" refers to a single-stranded nucleic acid comprised of two or more deoxyribonucleotides, such as primers, probes, nucleic acid fragments to be detected and nucleic acid controls. The exact size of an oligonucleotide depends on many factors and the ultimate function or use of the oligonucleotide. Oligonucleotides can be prepared by any suitable method, including, for example, cloning and restriction of appropriate sequences and direct chemical synthesis by a method such as the phosphotriester method of Narang et al., Meth. Enzymol. 68, 90-99 (1979); the phosphodiester method of Brown et al., Meth. Enzymol. 68, 109-151 (1979); the diethylphosphoramidite method of Beaucage et al., Tetrahedron Lett. 22, 1859-1862 (1981); and the solid support method described in the U.S. Pat. No. 4,458,066.
The term "primer" refers to an oligonucleotide, whether natural or synthetic, capable of acting as a point of initiation of DNA synthesis under conditions in which synthesis of a primer extension product complementary to a nucleic acid strand is induced, i.e. in the presence of the different nucleoside triphosphates and an agent for polymerization (e.g. a DNA polymerase or a reverse transcriptase) in an appropriate buffer and at a suitable temperature. The appropriate length of a primer depends on the intended use of the primer but typically ranges from 15 to 40 nucleotides. Short primer molecules generally require lower temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with a template and serve to initiate DNA synthesis under the chosen reaction conditions.
The term "primer" may refer to more than one primer, particularly in the case where there is some ambiguity in the information regarding one or both ends of the target region to be amplified. If a conserved region shows significant levels of polymorphism in a population, mixtures of primers can be prepared that will amplify such sequences, or the primers can be designed to amplify even mismatched sequences. A primer can be labeled, if desired, by incorporating a label detectable by radiometric, spectroscopic, photochemical, biochemical, immunochemical or chemical means. For example, useful labels include .sup.32 P, fluorescent dyes, electron-dense reagents, enzymes (as commonly used in ELISAs), biotin or haptens and proteins for which antisera or monoclonal antibodies are available. A label can also be used to "capture" the primer, so as to facilitate the immobilization of either the primer or a primer extension product, such as amplified DNA, on a solid support.
The term "probe" refers to an oligonucleotide containing a hybridizing sequence complementary to a part of the target sequence which may be labeled with a tag or attached to a solid support. Depending on the assay methods utilized for detecting hybrids formed between probes and nucleic acid sequences the probes may contain additional features in addition to the hybridizing region. In the dot blot format, for example, the probes are typically labeled. If the probe is first immobilized, as in the "reverse" dot blot format described below, the probe can also contain long stretches of poly-dT that can be fixed to a nylon support by irradiation, a technique described in more detail in PCT Patent Publication No. 89/11548. The appropriate length of a probe depends on the intended use of the probe but typically ranges from 15 to 40 nucleotides. Short probe molecules generally require lower temperatures to form sufficiently stable hybrid complexes with the target. A probe need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with the target sequence.
The probes of the present invention may be synthesized and labeled using the techniques described above for synthesizing oligonucleotides. For example, the probe may be labeled at the 5'-end with .sup.32 P by incubating the probe with .sup.32 P-ATP and kinase. A suitable non-radioactive label for probes is horseradish peroxidase (HRP). Methods for preparing and detecting probes containing this label are described in U.S. Pat. Nos. 4,914,210 and 4,962,02. For additional information on the use of such labeled probes, see U.S. Pat. No. 4,789,630; Saiki et al., N. Eng. J. Med. 319, 537-541 (1988); and Bugawan et al., Bio/Technology 6, 943-947 (1988). Useful chromogens include red leuco dye and 3,3',5,5'-tetramethylbenzidine (TMB). Helmuth, PCR Protocols, San Diego, Calif., Academic Press, Inc., 1990, pp. 119-128, describes procedures for non-isotopic detection of PCR products.